Plasma etching has become an important process in the fabrication of semiconductor devices in integrated circuits. A key requirement of any plasma process used in fabrication of semiconductor devices is that the devices formed must have uniform and consistent electrical characteristics across a wafer. In other words, the plasma etching process must be capable of creating highly uniform device structures with respect to thickness, dimensions and cross-sectional profiles. However, such uniformity has become increasingly difficult to obtain as wafers are increasing in diameter and device dimensions are becoming smaller. Combined with the need to lower the cost of production, plasma etch reactors must have improved uniformity and throughput.
Uniformity of device structures across a wafer is dependent upon factors such as the manner in which gas is introduced into the chamber and the temperature gradient across a wafer. In order to achieve uniform etching across a wafer, etching gas must be introduced into the plasma reactor in a uniform manner. Traditionally, this is accomplished through the use of a plurality of small inlet holes through which etching gas flows into a reaction chamber. Not only are the inlet holes difficult to machine, but also create uniformity problems on the wafer. The inlet holes introduce gas into a reaction chamber in a localized manner with the highest density of gas in areas near the holes. Since, the gas is not evenly introduced, the etch uniformity across a wafer varies significantly depending on the size and the number of inlet holes used.
Beside the gas delivery system, the amount of temperature gradient on a wafer also affects etch uniformity. Some plasma processes such as certain metal etch require wafers to be heated during the plasma process. To keep the temperature across a wafer uniform, the plasma etch system must be able to reduce the amount of heat loss or heat transfer from the wafer chuck on which the wafer sits. Typically, during plasma processing, only the top surface of a wafer chuck in a plasma reactor is under vacuum while the bottom surface of the chuck is typically at atmospheric pressure. The fact that the bottom surface of the wafer chuck is at atmospheric pressure causes heat loss through convection underneath the chuck. Not only does the convection heat loss decrease the temperature uniformity across a wafer during processing, it also reduces throughput because the heater takes longer to heat up a wafer prior to etching. Therefore, there is a need to develop a means to reduce heat loss from the bottom surface of the wafer chuck so as to increase etch uniformity and throughput.